This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The hallmark feature of tumor cells is a highly unstable genome. Bloom syndrome (BS), an autosomal recessive disorder that results from mutation of the BLM gene, exhibits extraordinarily high levels of sister chromatid exchange (SCE) events, a marker of genomic instability. BLM protein may influence genome stability through 3'-5'DNA helicase activity that can stabilize stalled replication forks caused by damage to the DNA. Since many classes of genotoxic agents have been shown to block replication, this information is essential to the understanding of the cellular responses to genotoxic agents. The homologous recombinational repair (HRR) pathway is required for SCE formation and restoration of a collapsed replication fork. Therefore, HRR is essential in maintaining genomic stability. Rad51, a protein central to the HRR pathway, physically interacts with BLM and thus could play a role in the elevated levels of SCE events seen in BS cells. The goal of this proposal is to define the amino acid residues of BLM physically interacting with Rad51 and to determine the complex's function as a molecular switch through which the cell can govern pathway choice. Knowledge of the physical parameters will assist in the determination of the functional significance of complex formation. The experiments proposed will address the following aims: (1) Refine the BLM amino acid sequence responsible for mediating complex formation with Rad51 through systematic deletion of 25 amino acid increments from the termini;and (2) Measure the biochemical kinetics of BLM-Rad51 complex formation.